Bonded substrate and a manufacturing method thereof, and a surface acoustic wave device using the said bonded substrate

ABSTRACT

[Object] An object of the present invention is to provide a bonded substrate which is excellent in temperature characteristics and suppresses unnecessary response due to reflection of an elastic wave at a bonding interface. 
     [Means to Solve the Problems] The present invention is unique in that a bonded substrate is constructed by bonding a LiTaO 3  substrate and a base plate wherein a Li concentration at a base plate-bonding face of the LiTaO 3  substrate is higher than that at a LiTaO 3  substrate-side end face of the bonded substrate, that the difference between the Li concentration at the base plate-bonding face of the LiTaO 3  substrate and the Li concentration at the LiTaO 3  substrate-side end face of the bonded substrate is 0.1 mol % or greater, that the Li concentration at the base plate-bonding face of the LiTaO 3  substrate satisfies an equation Li/(Li+Ta)×100=(50+α) mol %, where α is in the range of −1.2&lt;α&lt;0.5, that the Li concentration at the LiTaO 3  substrate-side end face of the bonded substrate satisfies an equation Li/(Li+Ta)×100=(48.5+β) mol %, where β is in the range of −0.5&lt;β&lt;0.5, and that the thickness measured from the base plate-bonding face of the LiTaO 3  substrate to the LiTaO 3  substrate-side end face of the finished bonded substrate becomes greater than 5 times but less than 20 times the wavelength of the surface acoustic wave or that of the leaky surface acoustic wave.

TECHNICAL FIELD

The present invention relates to a bonded substrate which is a lithiumtantalate single crystal substrate or a lithium niobate single crystalsubstrate bonded to a base plate, and a method of manufacturing thesame, and the invention also relates to a surface acoustic wave deviceusing the same bonded substrate.

BACKGROUND ART

A SAW (Surface Acoustic Wave) device having a comb-like electrode forexciting a surface acoustic wave on a piezoelectric substrate is used asa component for frequency adjustment and selection of a mobile phone.

For this surface acoustic wave device, a piezoelectric material, such aslithium tantalate (LiTaO₃; LT) and lithium niobate (LiNbO₃; LN), is usedto make it, because piezoelectric materials meet the requirements ofsmallness in size, small insertion loss, and ability to stop passage ofunnecessary waves.

In recent years, the communication band used in mobile phones has beentending to have a narrower band-to-band spacing and a wider individualbandwidth, and consequently the surface acoustic wave device, whoseproperties undergo changes with varying temperature, is being requiredto have reduced tendency in such changes.

As an example of a piezoelectric material meeting such a requirement,for example, Non-IP Document 1 has reported that the fluctuation infrequency of a surface acoustic wave device due to temperature can bereduced by using a substrate in which lithium tantalate and sapphire arebonded to each other.

PRIOR ART DOCUMENTS Non-IP Publications

Non-IP Publication 1:

M. Miura, T. Matsuda, Y. Satoh, M. Ueda, O. Ikata, Y. Ebata, and H.Takagi, “Temperature Compensated LiTaO₃/Sapphire Bonded SAW Substratewith Low Loss and High Coupling Factor Suitable for US-PCS Application,”Proc. IEEE. Ultrason. Symp., pp. 1322-1325, 2004.

SUMMARY OF THE INVENTION Problems to be Solved by Invention

However, in the substrate reported in non-IP Publication 1, since twomaterials having different acoustic impedances are combined, thereoccurs a problem that the elastic wave excited on the lithium tantalatesurface is reflected at the junction interface and becomes manifest asan unnecessary response. Accordingly, in the same document, it is shownthat the unnecessary response is decreased by increasing the thicknessof the lithium tantalate.

As a result of investigations conducted independently by the presentinventors, a problem was found that when the thickness of the lithiumtantalate of the bonded substrate is increased, the value Q of thesurface acoustic wave resonator or the pseudo surface acoustic waveresonator, which is the basic element of the surface acoustic wavedevice, drops to a level of normal (non-crystal) lithium tantalate.

It is therefore an object of the present invention to provide a bondedsubstrate having excellent temperature characteristics and high value ofQ, a method of manufacturing the same, and a surface acoustic wavedevice using this bonded substrate.

The inventors of the present invention diligently studied about theLiTaO₃ substrate constituting the bonded substrate, and as a result theydiscovered that when the Li concentration at the bonding face of theLiTaO₃ substrate is larger than the Li concentration on the exposedsurface side of the LiTaO₃ substrate, energy is confined in thevicinities of the exposed surface of the LiTaO₃ substrate (a region witha small Li concentration) where the acoustic wave velocity is lower dueto the tendency of the acoustic wave to go to a region with smaller Liconcentration (a region with a low velocity), so that the acoustic waveenergy concentrates there, with a consequence that the value Q of theresonator is pushed up, and hence possessed the present invention.

Means for Solving the Problem

Accordingly, the present invention relates to a bonded substratecomprising a LiTaO₃ substrate and a base plate, wherein the Liconcentration at that face of the LiTaO₃ substrate which is in contactwith the base plate is higher than the Li concentration at a LiTaO₃substrate-side (exposed) end face of the composite bonded substrate.

Also, it is preferable that the Li concentration at the baseplate-bonding face of the LiTaO₃ substrate differs from the Liconcentration at the LiTaO₃ substrate-side end face of the compositebonded substrate by 0.1 mol % or greater.

The Li concentration at the base plate-bonding face of the LiTaO₃substrate is preferably such that it satisfies Li/(Li+Ta)×100=(50+α) mol%, where Li is Li concentration, Ta is Ta concentration, a is in therange of −1.2<α<0.5, and the Li concentration at the LiTaO₃substrate-side end face of the bonded substrate is preferably such thatit satisfies Li/(Li+Ta)×100=(48.5+β) mol %, where β is in the range of−0.5<β<0.5.

It is also preferable that the thickness of the LiTaO₃ substrate in thebonded substrate is greater than 5 times but less than 20 times thewavelength of the surface acoustic wave or that of the leaky surfaceacoustic wave.

It is also preferable that an area in which the Li concentrationsatisfies the above formula meant for the base plate-bonding face of theLiTaO₃ substrate of the present invention extends from this bonding faceof the LiTaO₃ substrate toward the LiTaO₃ substrate-side end face of thebonded substrate (that is, into the LiTaO₃ substrate) by a distance of0.1 through 4 times the wavelength of the surface acoustic wave or thatof the leaky surface acoustic wave.

It is furthermore preferable that the bonded substrate of the presentinvention has an area extending from the bonding face of the LiTaO₃substrate toward the LiTaO₃ substrate-side end face of the bondedsubstrate in which (area) the Li concentration decreases as the depthfrom the bonding face of the LiTaO₃ substrate increases; and it isfurther preferable that this area, in which the Li concentration thusdecreases, is formed to have a thickness of 1 through 5 times thewavelength of the surface acoustic wave or that of the leaky surfaceacoustic wave. In addition to this, the range of the Li concentration inthe LiTaO₃ substrate-side end face of the bonded substrate of thepresent invention is preferably formed over a span of 1 through 20 timesthe wavelength of the surface acoustic wave or that of the leaky surfaceacoustic wave from the LiTaO₃ substrate-side end face of the bondedsubstrate toward the bonding face of the LiTaO₃ substrate.

It is preferable that the crystal orientation of the LiTaO₃ substrate ofthe present invention is rotated from 36° Y through 49° Y cut, and alsothat the base plate used in the present invention is made of a materialselected from Si, SiC, Spinel, and sapphire.

Furthermore, the present invention is characteristic also in that, in abonded substrate composed of a LiNbO₃ substrate and a base plate, the Liconcentration at a base plate-bonding face of the LiNbO₃ substrate isgreater than the Li concentration at a LiNbO₃ substrate-side end face ofthe bonded substrate.

The method of manufacturing a bonded substrate according to the presentinvention is characteristic in that a base plate and a LiTaO₃ substratein which the Li concentration is greater at the substrate surface thaninside the substrate are bonded together, and that a surface layer atthe LiTaO₃ substrate-side end face of the bonded substrate which liesopposite the bonding face of the LiTaO₃ substrate is removed in a mannersuch that the Li concentration at the bonding face of the LiTaO₃substrate becomes greater than that at the LiTaO₃ substrate-side endface of the bonded substrate.

It is preferable if the bonded substrate of the present invention isused for surface acoustic wave device.

Effects of the Invention

According to the present invention, it is possible to improve the ValueQ of the bonded substrate for surface acoustic waves. Accordingly, itbecomes possible to increase the thickness of the piezoelectric layer tothereby facilitate the removal of the unnecessary response caused byreflective waves, so that it is now possible to provide a bondedsubstrate for surface acoustic wave which has a reduced loss and whichcan improve the suppression degree outside the band when formed into afilter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A diagram showing a model of Li content profiles of LiTaO₃substrates respectively constituting bonded substrates of Example 1 andExample 2.

FIG. 2 A diagram showing input impedance waveforms of resonators formedrespectively on a bonded substrate of Example 1 and a bonded substrateof Comparative Example 2.

FIG. 3 A diagram showing a result of measuring the values Q ofresonators formed respectively on a bonded substrate of Example 1 and abonded substrate of Comparative Example 2.

FIG. 4 A diagram showing input impedance waveforms of resonators formedrespectively on a bonded substrate of Example 2 and a bonded substrateof Comparative Example 1.

FIG. 5 A diagram showing a result of measuring the values Q ofresonators formed respectively on a bonded substrate of Example 2 and anormal LiTaO₃ substrate of Comparative Example 1.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail, but the present invention is not limited thereto.

The bonded substrate of the present invention is formed by bonding aLiTaO₃ substrate on a base plate. A known method may be used for bondingthe LiTaO₃ substrate on the base plate, but considering the base plate'sinfluence to bend the substrate, it is preferable to use the roomtemperature bonding method. Further, intervening layers made ofmaterials such as oSiO₂, SiO_(2±0.5), a-Si, p-Si, a-SiC, Al₂O₃ and thelike may be present between the LiTaO₃ substrate and the base plate.

The bonded substrate of the present invention is characteristic in thatthe Li concentration at the base plate-bonding face of the LiTaO₃substrate is higher than the Li concentration at the LiTaO₃substrate-side end face of the bonded substrate. Therefore, there is adifference in Li concentration in the thickness direction of the LiTaO₃substrate bonded to the base substrate.

In the case where an intervening layer is present between the LiTaO₃substrate and the base plate, the base plate-bonding face of the LiTaO₃substrate shall be deemed to mean the face of the LiTaO₃ substrate incontact with the intervening layer.

Such a LiTaO₃ substrate can be obtained by diffusing Li from the surfaceof the LiTaO₃ substrate by a known method such as vapor phase diffusionmethod, for example. Since the Li concentration of the thus obtainedsubstrate at the surface thereof would become large and the Liconcentration inside the substrate would be smaller, if the LiTaO₃substrate is machined by grinding or polishing or the like to expose theinside of the LiTaO₃ substrate and the non-machined surface is made tobe the base plate-bonding face, a bonded substrate of the presentinvention is obtained.

Further, the LiTaO₃ substrate constituting the bonded substrate of thepresent invention may itself be a bonded substrate composed of aplurality of LiTaO₃ substrates. It is also possible to manufacture abonded substrate of the present invention by preparing a LiTaO₃substrate having a small Li concentration and a LiTaO₃ substrate havinga high Li concentration and bonding these to a base plate. For example,on one hand, as a LiTaO₃ substrate with a small Li concentration, aLiTaO₃ substrate of a congruent melt composition manufactured by a usualCzochralski method may be used, and on the other, as a LiTaO₃ substratehaving a large Li concentration, a LiTaO₃ substrate of a stoichiometriccomposition manufactured by a double crucible method may be used.

Further, the LiTaO₃ substrate constituting the bonded substrate of thepresent invention may be doped with a metal element such as Fe ifnecessary, or it may be subjected to a reduction treatment forsuppressing the pyroelectricity.

In the present invention, the difference between the Li concentration atthe base plate-bonding face of the LiTaO₃ substrate and the Liconcentration at the LiTaO₃ substrate-side end face of the bondedsubstrate is preferably 0.1 mol % or greater, or more preferably 0.25mol % or greater, or still more preferably 0.5 mol % or greater. Theupper limit of this difference is the difference between thestoichiometric composition (50±0.5 mol %) and the congruent meltcomposition (48.5±0.5 mol %), preferably 2.5 mol % or less, morepreferably 2.0 mol % or less, still more preferably 1.5 mol % or less.

In the present invention, the Li concentration at the base plate-bondingface of the LiTaO₃ substrate is Li/(Li+Ta)×100=(50+α) mol %, where α isin the range of −1.2<α<0.5, preferably in the range of −1.0<α<0.5, morepreferably in the range of −0.8<α<0.5, or still more preferably in therange of −0.8<α≤0.

From the viewpoints of easiness in manufacturing and manufacturing cost,the LiTaO₃ substrate constituting the bonded substrate of the presentinvention is preferably manufactured by first preparing a LiTaO₃substrate having a congruent melt composition through an ordinaryCzochralski method and then applying it to a Li diffusion treatment bymeans of a gas phase method. When the LiTaO₃ substrate is produced inthis way, the Li concentration at the substrate surface becomesLi/(Li+Ta)×100=(50+α) mol % where α is in the range of −1.2<α<0.5, whichis close to the stoichiometric composition.

The longer the time of applying the Li diffusion treatment is, the morelikely warpage and cracking would occur in the substrate, so that it ispreferable that the area wherein the Li concentration satisfiesLi/(Li+Ta)×100=(50+α) mol % where α is in the range of −1.2<α<0.5extends from the bonding face of the LiTaO₃ substrate toward the LiTaO₃substrate-side end face of the bonded substrate by a limited distance ofabout 0.1 through 4 times the wavelength of the surface acoustic wave orthe leaky surface acoustic wave. By doing so, it is possible to suppresswarping and cracking of the substrate and to increase the value Q of thebonded substrate.

Further, in the present invention, it is preferable to have an area inwhich the Li concentration decreases as the measurement moves from thebonding face of the LiTaO₃ substrate toward the LiTaO₃ substrate-sideend face of the bonded substrate. Such a bonded substrate can bemanufactured by using, as the LiTaO₃ substrate of a bonded substrate, aLiTaO₃ substrate having been subjected to a Li diffusion treatment by agas phase method. In this case, if a bonded substrate in which the Liconcentration profile has a steep variation is used, although it ispossible to increase the Value Q of the resonator, there is apossibility that a response noise is generated due to a steep Liconcentration variation. For this reason, it is preferable that the areain which the Li concentration decreases from the bonding face of theLiTaO₃ substrate toward the LiTaO₃ substrate-side end face of the bondedsubstrate spans over 1 to 5 times the wavelength of the surface acousticwave or the leaky surface acoustic wave. In this way, it is possible notonly to manufacture the device easily but also to suppress the responsenoise caused by the steep Li concentration change.

In the bonded substrate of the present invention, it is preferable that,on one hand, the Li concentration at the bonding face of the LiTaO₃substrate satisfies Li/(Li+Ta)×100=(50+α) mol % where α is in the rangeof −1.2<α<0.5, and, on the other hand, the Li concentration at theLiTaO₃ substrate-side end face of the bonded substrate satisfiesLi/(Li+Ta)×100=(48.5+β) mol % where β is in the range of −0.5<β<0.5, andalso that the LiTaO₃ substrate is of a congruent melt composition.

As described above, from the viewpoints of easiness in manufacturing andmanufacturing cost, the bonded substrate of the present invention ispreferably manufactured based on a LiTaO₃ substrate having a congruentmelt composition obtained through an ordinary Czochralski method andhaving been subjected to a Li diffusion treatment by means of a gasphase method. This way, the Li concentration inside the LiTaO₃ substratebecomes Li/(Li+Ta)×100=(48.5+β) mol % (wherein −0.5<β<0.5), which isclose to the congruent melt composition; and after bonding this LiTaO₃substrate to a base plate, the LiTaO₃ substrate is machined by grindingor polishing or the like to expose the inside of the LiTaO₃ substrate,whereby it is possible to obtain a bonded substrate wherein the Liconcentration at the LiTaO₃ substrate-side end face of the bondedsubstrate is Li/(Li+Ta)×100=(48.5+β) mol % where β is in the range of−0.5<β<0.5.

In addition, the area in which the Li concentration satisfiesLi/(Li+Ta)×100=(48.5+β) mol % where β is in the range of −0.5<β<0.5 canbe arbitrarily determined. However, if this area is narrowed relative tothe thickness of the LiTaO₃ substrate constituting the bonded substrate,it becomes necessary to lengthen the Li diffusion treatment time, with aconsequence that warping and cracking are more likely to occur in thesubstrate; so this area of the said Li concentration is preferablysubstantially thick. It is preferable that said area is formed to extendfrom the LiTaO₃ substrate-side end face of the bonded substrate towardthe base plate-bonding face of the LiTaO₃ substrate over about 1 to 20times the wavelength of the surface acoustic wave or the leaky surfaceacoustic wave.

The thickness of the LiTaO₃ substrate in the bonded substrate of thepresent invention is preferably greater than 5 times but less than 20times the wavelength of the surface acoustic wave or that of the leakysurface acoustic wave. In this way, it is possible to suppress anunnecessary response caused by the reflection of the acoustic wave atthe bonding interface.

In the present invention, the wavelength of the surface acoustic wave orthe leaky surface acoustic wave are, respectively, the wavelength of thesurface acoustic wave or the leaky surface acoustic wave when the bondedsubstrate is used as the surface acoustic wave device, and they aredetermined by the frequency of the electric signal input to the bondedsubstrate (surface acoustic wave device) and the velocity of the surfacewave (leaky wave). The velocity of the surface wave varies depending onthe material and is about 4000 m/s for LiTaO₃. Therefore, in the case ofmanufacturing a surface acoustic wave device for 1 GHz from a compositesubstrate using LiTaO₃ as the piezoelectric single crystal substrate,the wavelength of the surface acoustic wave becomes about 4 μm. In thecase of manufacturing a surface acoustic wave device for 2 GHz, thewavelength of the surface acoustic wave is about 2 μm, and in the caseof manufacturing a surface acoustic wave device for 800 MHz, thewavelength of the surface acoustic wave is about 5 μm.

Although the crystal orientation of the LiTaO₃ substrate constitutingthe present invention can be arbitrarily selected, it is preferable,from the viewpoint of characteristics, that it is 36° rotated Y-cutthrough 49° rotated Y-cut. Also, the material to make the plate used asthe base plate is not particularly limited, but it is preferablyselected from Si, SiC, spinel and sapphire.

Even if a LiNbO₃ substrate is used in place of the LiTaO₃ substrateconstituting the present invention, the value Q can be improvedsimilarly as in the case of the bonded substrate using the LiTaO₃substrate.

Accordingly, the present invention is also characteristic in that, in abonded substrate composed of a LiNbO₃ substrate and a base plate bondedtogether, the Li concentration at a base plate-bonding face of theLiNbO₃ substrate is greater than the Li concentration at a LiNbO₃substrate-side end face of the bonded substrate.

In addition, the difference between the Li concentration at the baseplate-bonding face of the LiNbO₃ substrate and the Li concentration atthe LiNbO₃ substrate-side end face of the bonded substrate is preferably0.1 mol % or greater.

It is preferable that the Li concentration at the base plate-bondingface of the LiNbO₃ substrate satisfies Li/(Li+Nb)×100=(50+α) mol % whereα is in the range of −1.2<α<0.5, and that the Li concentration at theLiNbO₃ substrate-side end face of the bonded substrate satisfiesLi/(Li+Nb)×100=(48.5+β) mol % where β is in the range of −0.5<β<0.5.

Furthermore, the thickness of the LiNbO₃ substrate in the bondedsubstrate is preferably greater than 5 times but less than 20 times thewavelength of the surface acoustic wave or that of the leaky surfaceacoustic wave.

It is also preferable that an area in which the Li concentrationsatisfies the above formula meant for the base plate-bonding face of theLiNbO₃ substrate of the present invention extends from this bonding faceof the LiNbO₃ substrate toward the LiNbO₃ substrate-side end face of thebonded substrate by a distance of 0.1 through 4 times the wavelength ofthe surface acoustic wave or that of the leaky surface acoustic wave.

It is furthermore preferable that the bonded substrate of the presentinvention has an area extending from the bonding face of the LiNbO₃substrate toward the LiNbO₃ substrate-side end face of the bondedsubstrate in which (area) the Li concentration decreases as the depthfrom the bonding face of the LiNbO₃ substrate increases; and it isfurther preferable that this area, in which the Li concentration thusdecreases, is formed to have a thickness of 1 through 5 times thewavelength of the surface acoustic wave or that of the leaky surfaceacoustic wave. In addition to this, the range of the Li concentration inthe LiNbO₃ substrate-side end face of the bonded substrate of thepresent invention is preferably formed over a span of 1 through 20 timesthe wavelength of the surface acoustic wave or that of the leaky surfaceacoustic wave from the LiNbO₃ substrate-side end face of the bondedsubstrate toward the bonding face of the LiNbO₃ substrate.

It is preferable that the crystal orientation of the LiNbO₃ substrate ofthe present invention is rotated from 0° Y through 30° Y cut or 128°±5°Y cut, and also that the base plate used in the present invention ismade of a material selected from Si, SiC, Spinel, and sapphire.

It is desirable to fabricate a surface acoustic wave device using one ofthese bonded substrates of the present invention, because a resonatorhaving a large value of Q can be constructed.

The bonded substrate according to the present invention can bemanufactured in the following manner: a base plate and a LiTaO₃substrate in which the Li concentration is greater at the surface of thesubstrate than inside the substrate are bonded together; a surface layerat the LiTaO₃ substrate-side end face of the bonded substrate which liesopposite the bonding face of the LiTaO₃ substrate is removed in a mannersuch that the Li concentration at the bonding face of the LiTaO₃substrate becomes greater than that at the LiTaO₃ substrate-side endface of the bonded substrate. On this occasion, it is also possible tobond a base plate on each one of the faces of a LiTaO₃ substrate inwhich the Li concentration is greater at the surface than inside thesubstrate, and then divide the LiTaO₃ substrate in half acrossthickness-wise middle plane, to thereby obtain bonded substrates. Inthis way, two bonded substrates can be obtained from one LiTaO₃substrate, which is also desirable from the viewpoint of cost.

Such a manufacturing method can similarly be applied to the case of aLiNbO₃ substrate.

The Li concentration of the LiTaO₃ substrate or the LiNbO₃ substrateconstituting the bonded substrate of the present invention may bemeasured by a known method, for example, by Raman spectroscopy. In thecase of LiTaO₃ single crystal, it is known that there exists a roughlylinear relationship between the half width of the Raman shift peak andthe Li concentration (Li/(Li+Ta) value) (See 2012 IEEE InternationalUltrasonics Symposium Proceedings, Page(s): 1252-1255, Applied Physics A56, 311-315 (1993)).

Therefore, by using a formula expressing such a relationship, it ispossible to evaluate the composition at an arbitrary position in anoxide single crystal substrate.

A formula expressing a relationship between the half width of the Ramanshift peak and the Li concentration can be obtained by measuring theRaman half width of a number of samples whose Li concentrations aredifferent and whose compositions are known; if the conditions of theRaman measurement are identical, it is also possible to use a formulathat has been already publicized such as in a literature.

For example, in the case of lithium tantalate single crystal, thefollowing Formula (1) may be used (See IEEE International UltrasonicsSymposium Proceedings, Page(s): 1252-1255).

Li/(Li+Ta)=(53.15−0.5 FWHM1)/100   (1)

wherein, “FWHM1” is the full width at half maximum of the Raman shiftpeak around 600 cm⁻¹.

Please refer to the literature for details of measurement conditions.

The value Q of the SAW resonator formed on the bonded substrate of thepresent invention and that of the SAW resonator of a comparative examplewere obtained through the following Formula (2) described in p. 861 ofthe publicized literature “2010 IEEE International Ultrasonics SymposiumProceedings Page(s): 861-863”. This equation appears as formula (1) inthe said literature, but since it overlaps with the above-describedformula number, it is given a formula number (2) in this specification.

Q(f)=ω*τ(f)*|Γ|/(1−|Γ|²)

wherein, ω is the angular frequency, τ(f) is the group delay time, and Γis the reflection coefficient measured by a network analyzer.

EXAMPLES

Hereinafter, examples of the present invention and comparative exampleswill be described more specifically.

Example 1

In Example 1, first, a singly polarized 4-inch diameter LiTaO₃ singlecrystal ingot having a roughly congruent composition and having a Li:Taratio of 48.4:51.6 was sliced to obtain a number of 370-μm-thick 42°rotated Y-cut LiTaO₃ substrates. Thereafter, in view of a circumstance,the surface roughness of each sliced wafer was adjusted to 0.15 μm inarithmetic average roughness Ra value by a lapping procedure, and theafter-finish thickness was set to 350 μm.

After the front and back faces of each resulting substrate were finishedinto a quasi-mirror surface having a Ra value of 0.01 μm by planarpolishing, the substrate was buried in a powder containing Li, Ta and Owith Li₃TaO₄ as a main component. On this occasion, the powder in whichLi₃TaO₄ was a main component was prepared by mixing Li₂CO₃ powder andTa₂O₅ powder at a molar ratio of 7:3, followed by baking at 1300° C. for12 hours. Then, this powder containing Li₃TaO₄ as a main component waslaid in a small container, and a plurality of said sliced wafers wereburied in this Li₃TaO₄ powder.

Then, this small container was set in an electric furnace, which wassubsequently filled with an N₂ atmosphere and heated at 900° C. for 20hours, to thereby cause Li to diffuse into the sliced wafer from thesurface toward the middle thereof. Thereafter, during the temperaturelowering stage of this diffusion treatment, the thus treated slicesubstrate was subjected to an anneal treatment at 800° C. for 12 hours,and during the subsequent stage where the wafer was allowed to coolfurther from 770° C. to 500°, an electric field of 4000 V/m was appliedin roughly +Z axis direction, and thereafter a treatment was conductedto cause the temperature to fall to the room temperature. Incidentally,after the application of the electric field, the furnace atmosphere maybe changed to be the atmosphere.

Also after this treatment, a rougher side face of each wafer wassubjected to a sandblasting to finish it to an Ra value of about 0.15μm, and the other quasi-mirror side face thereof was polished 3 μm deepand thus a plurality of LiTaO₃ single crystal substrates were obtained.

Next, the thus obtained LiTaO₃ substrate and a 230 μm-thick Si substratewere bonded together by a room temperature bonding method described in“Takagi H. et al, “Room-temperature wafer bonding using argonbeamactivation” From Proceedings—Electrochemical Society (2001), 99-35(Semiconductor Wafer Bonding: Science, Technology, and Applications V),265-274.” whereby a number of bonded substrates were obtained.

Specifically, a cleaned substrate was set in a high vacuum chamber; ahigh-speed atomic beam of argon whose ion beam has been neutralized isirradiated on the surface of the substrate to activate it (activationtreatment); thereafter, the LiTaO₃ single crystal substrate was bondedto the Si base plate. The LiTaO₃ substrate-side end face of the thusobtained bonded substrate was subjected to grinding and polishing in amanner such that what remained of the LiTaO₃ substrate on the bondinginterface of this bonded substrate came to have a thickness of 28 μm,and thus a bonded substrate consisting of a rotated Y-cut LiTaO₃substrate diffused with Li and the Si base plate was prepared.

Next, with respect of one of the thus prepared bonded substrates, ormore particularly with respect to the middle portion of the bondedsubstrate, a laser Raman spectrometer (LabRam HR series manufactured byHORIBA Scientific Inc., Ar ion laser, spot size 1 μm, room temperature)was used to measure the half-value width “FWHM1” of the Raman shift peakaround 600 cm⁻¹, which is an indicator of the Li diffusion amount, themeasurement proceeding in a depth-wise direction from the LiTaO₃-sidesurface, and the Li concentration was calculated using theabove-described Formula (1); and as the result a Li concentrationprofile as shown in FIG. 1 was obtained.

According to the result shown in FIG. 1, regarding this bondedsubstrate, at the base plate-bonding face of the LiTaO₃ substrate the Liconcentration was 49.6 mol % and the α value was −0.4 and at the LiTaO₃substrate-side end face of the bonded substrate the Li concentration was48.4 mol % and the β value was −0.1, so that it was confirmed that theLi concentration at the base plate-bonding face of the LiTaO₃ substratewas higher than the Li concentration at the LiTaO₃ substrate-side endface of the bonded substrate. Further, the difference between the Liconcentration at the base plate-bonding face of the LiTaO₃ substrate andthe Li concentration at the LiTaO₃ substrate-side end face of the bondedsubstrate was 1.2 mol %.

In addition, with respect to the area ranging between 0 μm and about 0.5μm in depth as measured from the bonding interface toward the LiTaO₃substrate-side end face of the bonded substrate, the results indicatedLi/(Li+Ta)=49.6 mol % and a presence of pseudo stoichiometrycomposition. It was confirmed that the bonded substrate had in thevicinity of its surface layer a transition layer having a thickness ofabout 20 μm, in which the Li concentration decreased as the measurementmoved toward the LiTaO₃ substrate-side end face of the bonded substrate;it was also confirmed that there was an area ranging from the LiTaO₃substrate-side end face of the bonded substrate to a depth of about 8 μmwherein the Li concentration indicated that Li/(Li+Ta)=48.4 mol % and anexistence of a roughly congruent composition.

Further, when the warpage of this bonded substrate was measured by theinterference method using laser light, the value obtained was as largeas 200 μm, while cracks and chippings were not observed.

Next, on the LiTaO₃ substrate-side end face of the bonded substrate asobtained in this manner, an Al film having a thickness of 0.2 μm wasformed by means of a sputtering method, and at the same time, by meansof a photolithography using a g-line as the light source, a SAWresonator was formed. On this occasion, the Al electrode was shaped byRIE (reactive ion etching), and a mixed gas of BCl₃, Cl₂, CF₄, and N₂was used as the gas for this RIE.

At this time, the thickness of the LiTaO₃ substrate in the bondedsubstrate was 28 μm, which is seven times the wavelength of the surfaceacoustic wave or that of the leaky surface acoustic wave, for the latterwere roughly 4 μm.

Then, the thus fabricated evaluation purpose SAW resonator was measuredby an RF prober for its property and the measurement result was as shownin FIG. 2. According to the result shown in FIG. 2, in the case ofExample 1, it was confirmed that a resonance waveform which is on thewhole good was obtained.

Next, the values of S11 and the group delay time as measured by theabove RF prober were input to a recording medium from a networkanalyzer, and the value Q was obtained by means of the above equation(2), and the result is shown in FIG. 3. According to the result shown inFIG. 3, the maximum value of Q was 1280.

Example 2

In Example 2, first, a singly polarized 4-inch diameter LiTaO₃ singlecrystal ingot having a roughly congruent composition and having a Li:Taratio of 48.75:51.25 was sliced to obtain a number of 370-μm-thick 42°rotated Y-cut LiTaO₃ substrates. Thereafter, in view of a circumstance,the surface roughness of each sliced wafer was adjusted to 0.15 μm inarithmetic average roughness Ra value by a lapping procedure, and theafter-finish thickness was set to 350 μm.

After the front and back faces of each resulting substrate were finishedinto a quasi-mirror surface having a Ra value of 0.01 μm by planarpolishing, the substrate was buried in a powder containing Li, Ta and Owith Li₃TaO₄ as a main component. On this occasion, the powder in whichLi₃TaO₄ was a main component was prepared by mixing Li₂CO₃ powder andTa₂O₅ powder at a molar ratio of 7:3, followed by baking at 1300° C. for12 hours. Then, this powder containing Li₃TaO₄ as a main component waslaid in a small container, and a plurality of said sliced wafers wereburied in this Li₃TaO₄ powder.

Then, this small container was set in an electric furnace, which wassubsequently filled with an N₂ atmosphere and heated at 900° C. for 10hours, to thereby cause Li to diffuse into the sliced wafer from thesurface toward the middle thereof. Thereafter, during the temperaturelowering stage of this diffusion treatment, the thus treated slicesubstrate was subjected to an anneal treatment at 800° C. for 12 hours,and during the subsequent stage where the wafer was allowed to coolfurther from 770° C. to 500°, an electric field of 4000 V/m was appliedin roughly +Z axis direction, and thereafter a treatment was conductedto cause the temperature to fall to the room temperature. Incidentally,after the application of the electric field, the furnace atmosphere maybe changed to be the atmosphere.

After this treatment, a bonded substrate was prepared in the same manneras in Example 1, and the LiTaO₃ substrate-side end face of the thusobtained bonded substrate was subjected to grinding and polishing in amanner such that what remained of the LiTaO₃ substrate on the bondinginterface of this bonded substrate came to have a thickness of 40 μm,and thus a bonded substrate consisting of a rotated Y-cut LiTaO₃substrate diffused with Li and the Si base plate was prepared.

Next, with respect of one of the thus prepared bonded substrates, ormore particularly with respect to the middle portion of this bondedsubstrate, a laser Raman spectrometer (LabRam HR series manufactured byHORIBA Scientific Inc., Ar ion laser, spot size 1 μm, room temperature)was used to measure the half-value width “FWHM1” of the Raman shift peakaround 600 cm⁻¹, which is an indicator of the Li diffusion amount, themeasurement proceeding in a depth-wise direction from the LiTaO₃-sidesurface, and the Li concentration was calculated using theabove-described Formula (1); and as the result a Li concentrationprofile as shown in FIG. 1 was obtained.

According to the result shown in FIG. 1, regarding this bondedsubstrate, at the base plate-bonding face of the LiTaO₃ substrate the Liconcentration was 49.4 mol % and the α value was −0.6 and at the LiTaO₃substrate-side end face of the bonded substrate the Li concentration was48.75 mol % and the β value was 0.25, so that it was confirmed that theLi concentration at the base plate-bonding face of the LiTaO₃ substratewas higher than the Li concentration at the LiTaO₃ substrate-side endface of the bonded substrate. Further, the difference between the Liconcentration at the base plate-bonding face of the LiTaO₃ substrate andthe Li concentration at the LiTaO₃ substrate-side end face of the bondedsubstrate was 0.65 mol %.

In addition, with respect to the area ranging between 0 μm and about 2μm in depth as measured from the bonding interface toward the LiTaO₃substrate-side end face of the bonded substrate, the results indicatedLi/(Li+Ta)=49.4 mol % and a presence of pseudo stoichiometrycomposition. It was confirmed that the bonded substrate had in thevicinity of its surface layer a transition layer having a thickness ofabout 8 μm, in which the Li concentration decreased as the measurementmoved toward the LiTaO₃ substrate-side end face of the bonded substrate;it was also confirmed that there was an area ranging from the LiTaO₃substrate-side end face of the bonded substrate to a depth of about 30μm wherein the Li concentration indicated that Li/(Li+Ta)=48.75 mol %and an existence of a roughly congruent composition.

Further, when the warpage of this bonded substrate was measured by theinterference method using laser light, the value obtained was as smallas 40 μm, while cracks and chippings were not observed.

Next, using this bonded substrate, a SAW resonator whose one-wave lengthwas about 4 μm was fabricated, and the thus fabricated evaluationpurpose SAW resonator was measured by an RF prober for its property andthe measurement result was as shown in FIG. 4. According to the resultshown in FIG. 4, in the case of Example 2 too, it was confirmed that aresonance waveform which is on the whole good was obtained.

At this time, the thickness of the LiTaO₃ substrate in the bondedsubstrate was 40 μm, which is 10 times the wavelength of the surfaceacoustic wave or that of the leaky surface acoustic wave, for the latterwere roughly 4 μm.

Next, the values of S11 and the group delay time as measured by theabove RF prober were input to the recording medium from the networkanalyzer, and the Value Q was obtained by means of the above equation(2), and the result is shown in FIG. 5. According to the result shown inFIG. 5, the maximum value of Q was 1380.

Comparative Example 1

In Comparative Example 1, a singly polarized 4-inch diameter LiTaO₃single crystal ingot having a roughly congruent composition and having aLi:Ta ratio of 48.4:51.6 was processed and lapped similarly as inExample 1, and a wafer having an after-finish thickness of 350 μm wasmade.

Next, after this lapping, the both faces of the wafer were finished byplanar polishing into mirror surfaces having a Ra value of 0.0001 μm,and thus a plurality of LiTaO₃ single crystal substrates having aroughly congruent composition were fabricated.

Then, similarly as in Example 1, with respect of one of the thusprepared LiTaO₃ single crystal substrate, or more particularly withrespect to the middle portion of this substrate, a laser Ramanspectrometer was used to measure the half-value width “FWHM1” of theRaman shift peak around 600 cm⁻¹, which is an indicator of the Lidiffusion amount, the measurement proceeding in a depth-wise directionfrom the surface, and the Li concentration was calculated using theabove-described Formula (1); and as the result it was found that the Liconcentration of this LiTaO₃ substrate remained roughly constant at 48.4mol % in the depth-wise direction and showed a roughly congruentcomposition.

Next, the warping of this substrate was measured by the interferencemethod using a laser beam, and the value obtained was as small as 40 μm,and cracks and chipping were not observed.

Next, on the surface of the substrate as obtained in this manner, an Alfilm having a thickness of 0.2 μm was formed by means of a sputteringmethod, and at the same time, by means of a photolithography using ag-line as the light source, a SAW resonator whose one-wave length wasabout 4 μm formed. On this occasion, the Al electrode was shaped by RIE(reactive ion etching), and a mixed gas of BCl₃, Cl₂, CF₄, and N₂ wasused as the gas for this RIE.

Then, the thus fabricated evaluation purpose SAW resonator was measuredby an RF prober for its property and the measurement result was as shownin FIG. 4. According to the result shown in FIG. 4, in the case ofComparative Example 1, it was confirmed that a resonance waveform whichis on the whole good was obtained.

Next, the values of S11 and the group delay time as measured by theabove RF prober were input to the recording medium from a networkanalyzer, and the value Q was obtained by means of the above equation(2), and the result is shown in FIG. 5. According to the result shown inFIG. 5, the maximum value of Q was 900, and this was lower than thevalues Q in the cases of Example 1 and Example 2.

Comparative Example 2

In Comparative Example 2, a singly polarized 4-inch diameter LiTaO₃single crystal ingot having a roughly congruent composition and having aLi:Ta ratio of 48.4:51.6 was processed similarly as in Example 1, and awafer having an after-finish thickness of 350 μm was made. Next,similarly as in Comparative Example 1, the both faces of this wafer werefinished by planar polishing into mirror surfaces having a Ra value of0.0001 μm, and thus a plurality of LiTaO₃ single crystal substrateshaving a roughly congruent composition were fabricated. Thereafter,similarly as in Example 1, this LiTaO₃ substrate was bonded to a230-μm-thick Si base plate; then the LiTaO₃ substrate-side end face ofthe thus obtained bonded substrate was subjected to grinding andpolishing in a manner such that what remained of the LiTaO₃ substrate onthe bonding interface of this bonded substrate came to have a thicknessof 28 μm, and thus a bonded substrate consisting of a rotated Y-cutLiTaO₃ substrate and the Si base plate was prepared.

Then, similarly as in Example 1, with respect of one of the thusprepared bonded substrate, or more particularly with respect to themiddle portion of this bonded substrate, a laser Raman spectrometer wasused to measure the half-value width “FWHM1” of the Raman shift peakaround 600 cm⁻¹, which is an indicator of the Li diffusion amount, themeasurement proceeding in a depth-wise direction from the LiTaO₃substrate-side end face, and the Li concentration was calculated usingthe above-described Formula (1); and as the result it was found that theLi concentration of this LiTaO₃ substrate constituting the bondedsubstrate remained roughly constant at 48.4 mol % in the depth-wisedirection and showed a roughly congruent composition so that the Liconcentration at the interface between the LiTaO₃ substrate and the baseplate was roughly equal to the Li concentration at the LiTaO₃substrate-side end face.

Next, the warping of this bonded substrate was measured by aninterference method using a laser beam, and the value obtained was assmall as 40 μm, and cracks and chipping were not observed.

Next, using this bonded substrate, a SAW resonator whose one-wave lengthwas about 4 μm was fabricated in the similar manner as in Example 1, andthe thus fabricated evaluation purpose SAW resonator was measured by anRF prober for its property and the measurement result was as shown inFIG. 2. According to the result shown in FIG. 2, in the case ofComparative Example 2, it was confirmed that a resonance waveform whichis on the whole good was obtained.

Next, the values of S11 and the group delay time as measured by theabove RF prober were input to the recording medium from a networkanalyzer, and the value Q was obtained by means of the above equation(2), and the result is shown in FIG. 3. According to the result shown inFIG. 3, the maximum value of Q was 1020, and this was lower than thevalues Q in the cases of Example 1 and Example 2.

As is confirmed from the comparison made above between Examples 1 and 2,on one hand, and Comparatives Examples 1 and 2, on the other hand, if abonding substrate of the present invention is used it is possible toobtain the effects arising from α value Q which is greater than isobtained in the case of a conventional LiTaO₃ substrate, and from thefact that there is scarce variation in Li concentration. Also, it wasconfirmed that when the bonding substrate of the present invention hasan a which satisfies −1.2<α≤−0.5, especially, and a β which satisfies−0.5<β<0.5, it is possible to obtain a bonded substrate wherein thevalue Q is further increased and the warping is reduced, and thus it ispossible to obtain a high value of Q favorable for surface acoustic wavedevice.

1. A bonded substrate comprising: a LiTaO₃ substrate; and a base platebonded to said LiTaO₃ substrate, wherein a Li concentration at a baseplate-bonding face of said LiTaO₃ substrate is higher than a Liconcentration at a LiTaO₃ substrate-side end surface of said bondedsubstrate.
 2. A bonded substrate as claimed in claim 1, wherein adifference between the Li concentration at the base plate-bonding faceof the LiTaO₃ substrate and the Li concentration at the LiTaO₃substrate-side end face of the bonded substrate is 0.1 mol % or greater.3. A bonded substrate as claimed in claim 1, wherein the Liconcentration at the base plate-bonding face of the LiTaO₃ substratesatisfies an equation Li/(Li+Ta)×100=(50+α) mol %, where α is in therange of −1.2<α<0.5.
 4. A bonded substrate as claimed in claim 1,wherein the Li concentration at the LiTaO₃ substrate-side end face ofthe bonded substrate satisfies an equation Li/(Li+Ta)×100=(48.5+β) mol%, where β is in the range of −0.5<β<0.5.
 5. A bonded substrate asclaimed in claim 1, wherein the thickness of the LiTaO₃ substrate in thebonded substrate is greater than 5 times but less than 20 times thewavelength of the surface acoustic wave or that of the leaky surfaceacoustic wave.
 6. A bonded substrate as claimed in claim 1, wherein anarea where Li concentration satisfies said equation for the baseplate-bonding face of the LiTaO₃ substrate expands from said bondingface toward the LiTaO₃ substrate-side end face of the bonded substrateby a thickness of 0.1 through 4 times the wavelength of the surfaceacoustic wave or that of the leaky surface acoustic wave.
 7. A bondedsubstrate as claimed in claim 1, wherein said bonded substrate possessesan area where Li concentration decreases as it is measured starting fromthe bonding face of the LiTaO₃ substrate toward the LiTaO₃substrate-side end face of the bonded substrate.
 8. A bonded substrateas claimed in claim 7, wherein said area where Li concentration thusdecreases is formed through a thickness of 1 through 5 times thewavelength of the surface acoustic wave or that of the leaky surfaceacoustic wave.
 9. A bonded substrate as claimed in claim 1, wherein anarea where Li concentration satisfies said equation for the LiTaO₃substrate-side end face of the bonded substrate expands from said LiTaO₃substrate-side end face of the bonded substrate toward the baseplate-bonding face of the LiTaO₃ substrate by a thickness of 1 through20 times the wavelength of the surface acoustic wave or that of theleaky surface acoustic wave.
 10. A bonded substrate as claimed in claim1, wherein a crystal orientation of said LiTaO₃ substrate is 36° rotatedY-cut through 49° rotated Y-cut.
 11. A bonded substrate as claimed inclaim 1, wherein said base plate is made of a material selected from Si,SiC, Spinel, and sapphire.
 12. A bonded substrate comprising: a LiNbO₃substrates and a base plate bonded to said LiNbO₃ substrate, wherein aLi concentration at a base plate-bonding face of said LiNbO₃ substrateis higher than a Li concentration at a LiNbO₃ substrate-side end surfaceof said bonded substrate.
 13. A surface acoustic wave device comprisingthe bonded substrate as defined in claim
 1. 14. A method formanufacturing a bonded substrate comprising: bonding a base plate to aLiTaO₃ substrate in which the Li concentration is greater at thesubstrate surface than inside the substrate; and removing a surfacelayer from the LiTaO₃ substrate-side end face of the bonded substratewhich lies opposite the bonding face of the LiTaO₃ substrate in a mannersuch that the Li concentration at the bonding face of the LiTaO₃substrate becomes greater than that at the LiTaO₃ substrate-side endface of the bonded substrate.
 15. A surface acoustic wave devicecomprising the bonded substrate as defined in claim 12.